With the establishment of the clean development mechanism (CDM) to cope with climate change and the regularization of the government support policy for new renewable energy development, an active effort has been made to recycle biogas from organic wastes, such as food waste, sewage/waste water sludge, animal/plant residue, animal carcass, butchery waste, etc., through anaerobic digestion.
In addition, the biogas plants are actively being constructed and operated to produce methane gas from combined digestion of food waste and livestock sludge for combined heat and power generation.
But, the biogas plants have the difficulty in producing biogas enough to secure economic feasibility, so an urgent attempt is being made to seek a new method of increasing the product yield of biogas.
As the actual operation and research on the anaerobic digestion technique for the production of biogas have long been performed, there has been remarkable progress in the anaerobic digestion mechanism, the optimal reaction conditions, and the operation method for digestion tank.
The pretreatment process for treating organic wastes prior to their addition into the digestion tank is still in its infancy, and it is possible to stabilize the operation of the digestion tank and reduce the size of the digestion tank. Also, it is reported that the pretreatment process leads to the increased yield of the biogas. Hence, various forms of the anaerobic digestion pretreatment technique have been developed and now commercially available.
On the other hand, a hydrolysis apparatus for organic waste using a chemical (e.g., hydrochloric acid) is developed based on the idea that the organic wastes need to be converted into low molecular organic substances directly digestible by microorganisms through hydrolysis in order to produce methane gas from the anaerobic digestion of the organic wastes.
There are two known fundamental methods to enhance the rate of the chemical reactions; increasing the reactive surface area and enhancing the agitation intensity.
For this reason, the existing chemical pretreatment methods or apparatuses for organic waste make the use of a separate two-step process of performing mechanical crushing and then a chemical reaction to increase the reactive surface area and a separated process of using an agitator or performing aeration and pumping circulation to enhance the agitation intensity.
KR utility model publication No. 20-0447884 (published on Mar. 3, 2010) discloses an “apparatus for liquefaction of animal carcass”, which is an apparatus for treating animal carcass using the above-described phenomenon with efficiency.
The “apparatus for liquefaction of animal carcass” is successful and now commercially available, as it completely liquefies animal carcass in a reaction time of about 6 hours or less and shows an effect of reducing the used amount of hydrochloric acid by about 30% and the cost of energy by about 30% or greater in comparison with the existing stationary apparatus for liquefaction as disclosed in KR patent publication No. 10-0865632 (published on Oct. 27, 2008) under the title of “a liquefied manure composition prepared using animal carcass and an apparatus for liquefaction of animal carcass”.
In addition, the alkali hydrolysis in Example 3 of “an amino acid manure removed of foul odor and its preparation method” according to KR patent publication No. 10-1331253 (published on Nov. 19, 2013) requires a greater amount of chemical by about 200% or more and takes a longer reaction time of about 11 hours.
FIG. 1 is an illustration of the above-mentioned apparatus for liquefaction of animal carcass according to the prior art. Referring to FIG. 1, the existing apparatus for liquefaction of animal carcass adopts an indirect heating construction that a reaction tank body 10 and a heating section 20 positioned under the reaction tank body 10 are separated from each other by a separation barrier, where the inner surface of the reaction tank body 10 is coated with a chemical-resistant material showing a resistance to a liquefying chemical 11.
With the use of the indirect heating construction, it is impossible to apply a thick chemical-resistant coating to the inner surface of the reaction tank body 10 in consideration of the heat transfer efficiency of the heating section 20.
To solve this problem, the conventional apparatus uses a ceramic coating, a Teflon coating, or a PP coating to the inner surface of the reaction tank body 10. But, the coated surface of the reaction tank body 10 is readily broken by the frictional abrasion as a result of the generation of shock wave and the movement of animal carcass 12 and more severely damaged by the liquefying chemical 12, leading to the need of replacing the whole apparatus.
As the ceramic coating of the reaction tank body 10 is particularly susceptible to shock, the indirect heating construction of FIG. 1 with a chemical-resistant coating based on a plastic material has the difficulty of applying high temperature to the heating section 20. Thus, the heating section 20 is necessarily maintained at low temperature of about 150° C. or below in consideration of the thermal stability and mechanical strengths of the plastic material.
It is therefore difficult to rapidly increase the internal temperature of the reaction tank body 10 and reduce the reaction time by raising the temperature of the heating section 20 or to manufacture a large-capacity industrial reaction tank that uses lots of energy for heating.
This may also be problematic in developing a pretreatment apparatus for liquefaction in a large-capacity facility for treatment of food waste and sewage/waste water sludge other than animal carcass.
Further, the hydraulic crushing and agitation shows a successful result when the inside of the reaction tank is spacious enough to allow the movement of a fluid, but otherwise nearly ineffective.
When the reaction tank is filled with bulky solids such as animal carcass, for example, it has lots of space between the solids and thus allows the free movement of the aqueous chemical solution, resulting in a great effect through hydraulic crushing and agitation even with a small amount of the chemical.
Contrarily, when the reaction tank is filled with small solids such as butchery waste consisting of pig hair, toenails, or the like and nearly free from space, it is impossible to induce the movement of a fluid or the generation of shock wave in the reaction tank filled with the chemical and driven to operation, nearly having no effect of liquefying the solids even with an elapse of considerably long time.
Besides, food waste and sewage/waste water sludge produced in a large quantity as industrial organic wastes cause the same problem as specified above, so it is difficult to use the existing reaction tank in the hydrolysis apparatus for liquefaction.
Lastly, the inside of the reaction tank body is present as one integrated space, so the concentration and temperature of the chemical become uniform all in the reaction tank on the instant.
Accordingly, the dilution of the chemical and the change of properties of the aqueous solution that occur as a result of the liquefaction have an instant effect to make the difference in the reaction rate.
For example, the liquefaction of frozen animal carcass in the winter season has a considerably long reaction time due to the low reaction rate.